You’re limited to one section of the city during Act 1, very much equal to White Orchard in W3. Once Act 2 begins the whole map is opened up for the taking.
So yeah it sort of it the prologue.
And it took me around 15 hours. Doing everything in that one city zone.
I never ran into the limits to my ability to roam actually, that's how big and full of content the part you're in is. Never realized I was gated in at all.